In modern angle measurement methods, magnetic field sensors are operated in a measuring bridge apparatus. An absolute angle measurement over an angular range of 360xc2x0 using AMR sensors is possible only at a high cost. With Hall or GMR sensors, it is possible to expand the range to 360xc2x0; with AMR sensors, a measurement range of 180xc2x0 is possible.
In modern magnetic field sensors, which contain a bridge circuit (for example, a Wheatstone bridge), a 360xc2x0 absolute angle measurement can only be achieved with difficulty, if at all. A sensor element supplies a voltage, which behaves in accordance with equation (1):
Uxcex8=U0+xcex94U cos2(xcex8)xe2x80x83xe2x80x83(1)
In AMR sensors, an absolute angle measurement of up to 180xc2x0 is consequently possible without additional electrical wiring. Measuring the absolute angle requires two full bridges, which are disposed offset from each other by 45xc2x0. One of these full bridges supplies a sine signal; the other full bridge supplies a 90xc2x0 phase-shifted cosine signal of the voltage curve to be measured.
If the output voltages are set in relation to each other, then the absolute angle of the phase shift can be determined by using the arctan function. The following equation is used for this purpose:                               θ          xe2x80x2                =                              1            2                    ⁢                      arctan            ⁡                          (                                                U                  ⁢                                      xe2x80x83                                    ⁢                                      sin                    ⁡                                          (                      θ                      )                                                                                        U                  ⁢                                      xe2x80x83                                    ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                                      (                    θ                    )                                                              )                                                          (        2        )            
The quadratic relation according to equation (1) permits only a saw-toothed function with a 180xc2x0 coverage.
A 180xc2x0-period angular resolution yields multiply defined values in the event of a full rotation (360 angular degrees). Often, however, functional considerations make a uniquely defined value range necessary or desirable. For example, this can be achieved by means of additional magnetic coils inside or outside the sensor element. Embodiments of this kind, however, require costly sensors and complex evaluation circuits. This additional wiring complexity and the attendant additional costs should be avoided. Moreover, sensors with magnetic coils can only be retrofitted into already existing circuits with difficulty.
With the embodiment proposed according to the invention, an absolute angle calculation can be carried out, which does not involve increased hardware requirements. In already existing evaluation circuits, which are equipped with an A/D converter, modifications can be carried out in the circuit in order to carry out the method according to the invention, which involve only a low retrofitting cost. Due to the degree of freedom in the design of the scaling factors Asin, Acos, the evaluation method can be adapted to the other sensors easily and with no trouble. The calculation of the offset values Ksin and Kcos can take place as part of band end programming, for example during the production of the evaluation system.
The embodiment proposed according to the invention permits the calculation of the offset values Ksin and Kcos to take place in both the analog part and the digital part of an evaluation circuit.
The method proposed according to the invention permits an extension of the arctan function from 180xc2x0 to 360xc2x0 and consequently an expansion of the value range. If the transitions of the 360xc2x0 flanks are detected, the method according to the invention also permits an incremental measurement or an absolute angle detection of those absolute angles that are greater than 360xc2x0. Even in this embodiment of the method according to the invention, additional magnetic coils, which are integrated into sensor elements, can be eliminated. This allows the embodiment variant to also be implemented in evaluation circuits, which have already been delivered or are already in operation, without high retrofitting costs. The method for expanding the absolute angle measurement range proposed according to the invention permits extremely precise absolute angles to be measured within a full rotation of 360xc2x0; the method proposed according to the invention also easily permits an increase of the measurement range by a multiple of a 360xc2x0 rotation.